There are some known limitations to the OnSend Addin types, generally. Please note the points below.
- The Outlook Addin is required for DLP detection on Mac OS 11.x or greater
- Windows machines do not need this add-in, they use a COM add-in for Outlook which is applied during agent installation
The majority of the limitations below apply to all 'On-Send' type of add-ins in Outlook, not just the Symantec DLP one.
- It only works for Office 365 subscription.
- Send Later feature is not available with onSend add-ins
- Only one ItemSend event is supported per add-in.
- OnSend Add-in is not available for group mailboxes.
- OnSend Add-ins cannot be published to AppSource.
- OnSend Add-ins does support Outlook in Offline mode
- OnSend does not work with mail marked 'Do Not Forward' and 'Encrypt Only' options
- OnSend does not work with AIP Encrypted Emails generated by selecting below Encrypt options:
- Confidential \ All Employees
- Highly Confidential \ All Employees
See Microsoft article (external) for additional details